1. Field of the Invention
This invention relates to an improvement in an oil pan for an internal combustion engine, and more particularly to an oil pan in which the oil dropped on a shallow bottom section is rapidly returned into a sump.
2. Description of the Prior Art
An internal combustion engine for an automotive vehicle is usually provided with an oil pan having a sump into which lubricating oil dropped in the oil pan is collected to be fed to various sections of the engine by an oil pump. In order to effectively return the lubricating oil into the sump, it has been proposed that an oil pan have a plurality of guide ribs on a shallow bottom section of the oil pan as shown in FIG. 6 of the present application. Such an arrangement is disclosed, for example, in Japanese Patent Publication No. 53-16048.
As shown in FIG. 6, an oil pan 21 includes a sump 22 disposed under a cylinder block (not shown), and extends along the axis of a crankshaft (not shown). A shallow bottom section 23 is disposed under the cylinder block and defines a space which communicates with the sump 22. An oil strainer (not shown) is disposed in the sump 22. A generally straight rib 24 is formed at the upper surface on one side of the shallow bottom section 23 which side is located downstream of the oil flow caused by the rotation of the crankshaft. The straight rib 24 extends generally in the longitudinal direction of the engine. A plurality of arcuate ribs 25 are formed at the upper surface on the side of the shallow bottom section 23 which side is located upstream of the oil flow caused by the rotation of the crankshaft. Lubricating oil in the shallow bottom section 23 is forced in the direction indicated by arrows A under the rotation of the crankshaft. This causes the lubricating oil to be rapidly returned from the shallow bottom section 23 into the sump 22. The shallow bottom section 23 is located considerably near the crankshaft since modern engines tend to be formed smaller.
However, with this conventional arrangement, lubricating oil guided by the ribs 25 flows into the sump section 22 through a flat section 26 which is formed between the rib 24 and end 25a of the each rib 25. Additionally, the lateral width of the flat section 26 gradually becomes narrower toward the sump 22, and the amount of the oil directed to the sump 22 increases on the flat section 26 as its approaches the sump 22. The oil on the flat section 26 overflows the ribs 24 and 25 in the vicinity of the sump 22 and is splashed by air flow caused by the high speed rotation of the crankshaft and the like, so that the oil is prevented from returning into the sump 22. Furthermore, since the flat section 26 is located just under the crankshaft with a short distance between them, the oil on the flat section 26 receives a strong air flow force caused by the high speed rotation of the crankshaft and the like, so that the oil is further prevented from returning into the sump 22.
Therefore, the lubricating oil tends to be splashed by the crankshaft and connecting rods. This splashing causes a low oil level in the sump 22, and therefore air is sucked into the oil flow circuit through the oil strainer. Thus, a large amount of air bubbles are mixed with the oil.